Nuclear power plants currently store their spent fuel assemblies on site for a period after being removed from the reactor core. Such storage is typically accomplished by placing the spent fuel assemblies in closely packed fuel racks located at the bottom of on site storage pools. The storage pools provide both radiation shielding and much needed cooling for the spent fuel assemblies.
Fuel racks often contain a large number of closely arranged adjacent storage cells wherein each cell is capable of accepting a spent fuel assembly. In order to avoid criticality, which can be caused by the close proximity of adjacent fuel assemblies, a neutron absorbing material is positioned within the cells so that a linear path does not exist between any two adjacent cells (and thus the fuel assemblies) without passing through the neutron absorbing material.
Early fuel racks utilized a layer of neutron absorbing material attached to the cell walls of the fuel rack. However, these neutron absorbing materials have begun to deteriorate as they have been submerged in water for over a decade. In order to either extend the period over which the fuel assemblies may be stored in these fuel racks, it is necessary to either replace the neutron absorber in the cell walls or to add an additional neutron absorber to the cell or the fuel assembly.
In an attempt to remedy the aforementioned problems with the deteriorating older fuel racks, the industry developed removable neutron absorbing inserts, such as the ones disclosed in U.S. Pat. No. 5,841,825 (the “'825 patent”), to Roberts, issued Nov. 24, 1998; U.S. Pat. No. 6,741,669 (the “'699 patent”), to Lindquist, issued May 25, 2004; and U.S. Pat. No. 6,442,227 (the “'227 patent”), to Iacovino, Jr. et al., issued Aug. 27, 2002. As of recent times, the neutron absorbing insert has become the primary means by which adjacent fuel assemblies are shielded from one another when supported in a submerged fuel rack. Thus, newer fuel racks are generally devoid of the traditional layer of neutron absorbing material built into the structure of the fuel rack itself that can degrade over time. Instead, fuel assembly loading and unloading procedures utilizing neutron absorbing inserts have generally become standard in the industry.
While the neutron absorbing inserts disclosed in the '825 patent, the '227 patent and the '699 patent have proved to be preferable to the old fuel racks having the neutron absorbing material integrated into the cell walls, these neutron absorbing inserts are less than optimal for a number of reasons, including without limitation complexity of construction, the presence of multiple welds, complicated securing mechanisms, and multi-layered walls that take up excessive space within the fuel rack cells. Additionally, with existing designs of neutron absorbing inserts, the inserts themselves must be removed prior to or concurrently with the fuel assemblies in order to get the fuel assemblies out of the fuel rack. This not only complicates the handling procedure but also leaves certain cells in a potentially unprotected state.
The '825 patent discloses a neutron absorbing apparatus which includes two adjacent neutron absorbing plates and a mounting assembly with latching means configured to be secured to fuel assemblies while the fuel assemblies remain under water in a fuel storage rack. The two neutron absorbing plates of the '825 patent are positioned orthogonally to form a chevron cross section which is placed about the fuel assemblies by insertion in the existing space between the fuel assemblies and the cell walls of a fuel storage rack. The primary embodiment of the neutron absorbing apparatus of the '825 patent utilizes a three layer configuration consisting of a backing plate (made of aluminum or stainless steel), a neutron absorbing sheet (made of cadmium, borated stainless steel, borated aluminum, or boron in a ceramic matrix), and a cover plate (made of aluminum or stainless steel). This multi-layer embodiment is both cumbersome and difficult to manufacture. Moreover, the absence of the neutron absorbing sheet at the fold in the backing plate and at the lateral edges of the backing plate is less than optimal and provides a potential area for increased reactivity.
It should be noted that the '825 patent also discloses a second embodiment of a neutron absorbing apparatus that allegedly eliminates any loss of nuclear absorber coverage at the fold in the backing plate and at the same time simplifies the fabrication process. In this embodiment, a special single-layer backing plate made of borated aluminum or borated stainless steel is used to replace the multi-layer arrangement of the primary embodiment. This special backing plate is itself a nuclear absorber and thus no additional absorber layer is added to provide the nuclear absorption. However, for this embodiment, the '825 patent teaches that the special backing plate must be formed by two plates arranged to form the chevron configuration and welded together at their juncture. In this regard, the '825 patent specifically states that for this embodiment “[t]he two individual plates are necessary because the borated backing plates cannot be folded, but must [be] welded. [T]he two borated backing plates . . . are welded together along [the] seam . . . to provide the chevron formation necessary to produce [the] plates . . . of the complete invention.” For obvious reasons, welds and joints in the body of the neutron absorbing apparatus are less than optimal.
Turning to the '227 patent, a sleeve assembly for refurbishing a fuel rack having cells in which fresh or spent nuclear fuel assemblies may be stored is disclosed. The sleeve assembly of the '227 patent has at least one elongate wall extending from the topside of a sleeve base having an opposed bottom side. The sleeve base has a flow hole extending therethrough that communicates with one of the rack base plate flow holes. A pin assembly disposed in the sleeve base flow hole has resilient tabs extending beyond the bottom side of the sleeve base for extending into a rack base plate flow hole and resiliently engaging the rack base plate when the sleeve assembly is installed in one of the cells. The pin assembly resists horizontal and vertical movements of the sleeve assembly, permits water flow into the cell and permits sleeve assembly removal tools and inspection devices to access the pin assembly.
The '227 patent discloses an embodiment of a sleeve assembly having chevron shaped walls formed by a single-plate. The '227 patent discloses that these walls are an extruded composite of boron carbide and aluminum. The extruding process to form the chevron shaped walls is believed to be less than optimal as it is difficult to perform, yields unpredictable result, requires extremely tight tolerances and results in an inferior product.
Turning now to the '669 patent, a neutron absorber system for a nuclear fuel storage rack is disclosed that includes a neutron absorber that is adapted to attach to a plurality of cell walls of a cell of the nuclear fuel storage rack. The neutron absorber is adapted to elastically deform. Means for applying at least one stress to the neutron absorber and means for releasing the at least one stress to cause the neutron absorber to attach to the plurality of cell walls of the cell of the nuclear fuel storage rack is also disclosed.
In one embodiment, the '669 patent teaches a multi-plate longitudinal weldment to form the body of the neutron absorber system. Specifically, the '669 patent teaches welding a metal matrix alloy corner piece to two metal matrix neutron absorber composite plates to form the chevron shape. Welds and joints in the body of the neutron absorbing apparatus are less than optimal. These welds in this embodiment render the neutron absorber system less than optimal.
The '669 patent also teaches a neutron absorber system having chevron-shaped walls that are formed of a metal composite which includes neutron absorbing material, for example, boron carbide or a metal boron alloy, such as aluminum, magnesium, titanium, aluminum/magnesium or aluminum/titanium, in combination with boron, for example. The '629 patent also discloses that the material may be stainless steel/boron alloys and that besides boron carbide and elemental boron, any element with a high thermal neutron absorption cross section may be substituted. The '669 patent further states generally that the first wall and the second wall of the chevron-shaped body “may be formed of a unitary material or they may be formed separately and attached to each other, for example, via standard TIG welding or by friction stir welding.” Despite this statement, the '669 patent is devoid of any enabling teaching as to how the chevron shaped body (which consists of the two walls connected along a longitudinal edge) is formed of a unitary material of a metal composite including a neutron absorbing material. Such materials tend to be very brittle when the percentage of boron carbide becomes substantial and thus, to date, it has been generally accepted in the art that only flat plates can be satisfactorily created from such materials. The only exception being the extruding process mentioned in the '227 patent, which as stated is less than optimal and undesirable. Therefore, the '669 patent also fails to teach a suitable neutron absorber insert and an enabling method of manufacturing such an insert.
These and other limitations of the prior art are overcome by the present invention which is described in the following detailed specifications.